xii I>K,I>I li\ 133 



iii^ to tin- 8 on the other. Kadi tooth consists of a crown, 

 and one or more fangs, which are embedded in sockets in the 

 bom- of tlu- jaw. The fangs Jire hollow and contain a vascular 

 tissue alled the pulp, and there is a hole at the bottom of 

 each fang by which a nerve and blood-vessels pass to the tooth. 

 The four teeth in the middle of each jaw, two on each side of 

 the middle line, have a chisel like crown and a single fang. 

 They are the incisors. Next to the two incisors in each half 

 of the jaw is a tooth with a more pointed crown and a single 

 fang, which resembles the long pointed teeth of a dog, and 

 is therefore called the canine. Next to this are two teeth 

 having a crown of two tubercles, and a fang partly divided 

 into two ; these are called the bicuspids. Next to these are 

 three teeth with large broad crowns and two or three fangs, 

 called the molars, or grinders. 



In childhood there is a temporary set of teeth called the 

 milk teeth, consisting of twenty teeth, two incisors, one canine, 

 and two molars on each side of each jaw. These are usually 

 all cut by the end of the second year. About the sixth or 

 *e\enth year the permanent teeth begin to appear and the 

 milk teeth gradually drop out. By the age of thirteen all 

 the permanent teeth are usually cut except the four last molars, 

 that is, the last tooth on each side of each jaw ; these are cut 

 much later than the other teeth, sometimes not till twenty-five 

 or thirty years of age, hence are called the wisdom teeth. 



Teeth are mainly composed of a substance called dentine, 

 a hard, calcified material like bone. Examined microscopically, 

 it is found that dentine does not contain lacuna. 1 and canaliculi 

 arranged in systems as seen in bone, but contains instead a 

 large number of minute tubules arranged parallel to one 

 another in the hard material. The crown of the tooth is 

 capped by a very hard substance called enamel, which con- 

 tains only 2 per cent of animal matter. The dentine of the 

 fangs is covered by a thin layer of bone called cement, and 

 this is fixed to the fibrous vascular periosteum which lines the 

 socket in the jaw bone by which the tooth is held. 



Mastication and Swallowing. Mastication or the 

 breaking up of the food by the teeth is carried out by 

 the mo\ i inciit ut the lower jaw working one set of teeth 

 against the other, up and down in cutting and biting, and 



